1) Field of the Invention
The present invention relates to an optical module and an optical communication system that can reduce an insertion loss in an optical transmission path.
2) Description of the Related Art
A conventional technology has been disclosed in, for example, Japanese Patent Application Laid-open No. 2003-163406. FIG. 20 illustrates how a conventional optical module 300 is mounted. Precisely, the optical module 300 is arranged between optical fibers 301 and 302. The optical module 300 includes lenses 303 and 304, an optical device 305, end portions of the optical fibers 301 and 302. The optical device 305 may be an optical modulator, an optical switch, or a directional optical coupler, having an optical waveguide.
The optical fiber 301 guides light to the optical device 305. Thus, the optical fiber 301 is provided on the “input side”. An output plane 301a of the optical fiber 301 is optically coupled with an input plane 300a of the optical device 305 via the lens 303.
The optical fiber 302 guides light away (or outputs) from the optical device 305. Thus, the optical fiber 302 is provided on the “output side”. An input plane 302a of the optical fiber 302 is optically coupled with an output plane 300b of the optical device 305 via the lens 304.
The light output from the output plane 301a of the optical fiber 301 is collected by the lens 303 and input to the input plane 300a of the optical device 305, and propagates in the optical waveguide (not shown) in the optical device 305.
The light output from the output plane 300b of the optical device 305 is collected by the lens 304 and input to the input plane 302a of the optical fiber 302, and propagates in the optical fiber 302.
The insertion loss when the optical module 300 is inserted between the optical fiber 301 and the optical fiber 302 is expressed by the following equation (1):Insertion loss=(coupling loss)+(loss in optical device)  (1).
In equation (1), the coupling loss stands for a sum of a loss accompanying optical coupling between the output plane 301a of the optical fiber 301 and the input plane 300a of the optical device 305, and a loss accompanying optical coupling between the output plane 300b of the optical device 305 and the input plane 302a of the optical fiber 302. The loss in optical device stands for a loss when the light propagates in the optical device 305.
Conventionally, the optical device 305 is made from lithium niobate. Lithium niobate has an advantage in that the coupling loss is as low as about 0.5 decibel, though increasing the size of the optical device 305, as compared with an optical device comprising a semiconductor.
The insertion loss of the lithium niobate optical device 305 is a sum of the coupling loss (about 0.5 decibel) and a loss in the optical device (about 0.5 decibel), which is about 1.0 decibel.
Recently, the optical device 305 is made from a semiconductor, instead of lithium niobate, in response to demands for a small and thin optical module 300 (optical device 305).
It is advantageous to use the semiconductor in that the optical device 305 (optical module 300) can be made small and thin, as compared with the lithium niobate optical device, but has a disadvantage in that the coupling loss reaches about 5.0 decibels. When the optical device 305 is made from the semiconductor, the insertion loss of the optical module 300 is a sum of the coupling loss (about 5.0 decibels) and the loss in the optical device (about 0.5 decibel), which reaches about 5.5 decibels.
In other words, if the optical device 305 is made of a semiconductor, there is a problem in that the insertion loss increases by about 5.0 decibels. The reason being, as the size becomes small, the optical mode size in the optical waveguide of the optical device 305 becomes smaller than that of the lithium niobate optical device, and hence a loss at the time of optical coupling increases.
The optical mode size refers to a half of the width when an electric field reaches the maximum value, 1/e, if it is assumed that the electric field distribution in the optical waveguide (including the optical fiber) is Gaussian.